The invention relates to a projector apparatus of the type including a light source and an aperture for an object, the image of which is to be projected, the aperture being placed in the ray path between a projection head and the light source and an ellipsoidal mirror being placed to have its focus points at the light source and the projection head, respectively.
A usual type of overhead projector includes a Fresnel mirror forming the projection stage or aperture for large transparencies, a light source and a projection head being situated centrally above the projection stage at the focal point of the Fresnel mirror.
This type of projector has several disadvantages. The Fresnel structure will be visible in the projected image, and there is further a considerable spread of light in the Fresnel structure which can cause a dazzling effect. In addition, the light must pass twice to the picture carrying material, such as a large transparency, thus emphasizing the effects and impurities in the material with the result that colour reproduction will be poor. The transparency should lie flat on the projection stage, since if there is a space between the material and the mirror there will be doubling effects in the projected image. These doubling effects are the reason why it is not possible to replace the Fresnel mirror with a corresponding concave mirror.
At least some of the above mentioned drawbacks could be mitigated or removed with an overhead projector comprising an ellipsoidal mirror, one focus of which coincides with the optical head and the other focus, of which coincides with the light source. Such a projector is known from CH No. 607 652. But the optical head in the known projector is centered on the aperture or stage of the projector. It is true that it has been suggested, in connection with another type of projector (DE-Al No. 3 443 356) with a lens between optical head and light source, to have the optical head and the light source displaced from central position in order to have the projected image moved upwardly, but the result is discouraging, because the colour errors become significant or unacceptable even for small displacements.
An object of the invention is therefore to provide a projector structure allowing placing of the transparency at a planar aperture for projecting an image substantially without distortion onto a screen parallel, or at right angles to the stage, with the projected image displaced in relation to the stage.
In accordance with the mention, a projector apparatus including a light source, a projection head and an aperture for an object, the image of which is to be projected, the aperture being in the ray path between the light source and the projection head, distinguishes over the prior art in that an ellipsoidal mirror is situated in the ray path between the light source and the projection stage aperture; in that one or a first focal point of the ellipsoidal mirror substantially coincides with the light source; and in that the other or a second focal point of the mirror substantially coincides with the projection head. The proper apparatus is characterized in that the projection head is positioned in a plane, which is orthogonal to the aperture and parallel with a tangent to the periphery of the aperture and in that the distance L between said plane and said tangent is substantially determined by the relation EQU 0.ltoreq.L.ltoreq.T/4
where T=the "height" of the aperture.
The surface of the aperture may be oriented substantially horizontally, the projection head being situated at a level above the aperture, the head being equipped with means known per se for deflecting the light into a generally horizontal direction. The projection head advantageously is situated in a plane normal to the aperture plane and containing the rear or forward edge of the aperture, as observed in the generally horizontal projection direction. If the projection head is then situated substantially in a plane constituting a plane normal to the aperture surface and coinciding with the forward edge of the aperture, the deflecting member should be a mirror, since the projector apparatus then functions as a genuine overhead projector, and such that its operator can place a transparency right side up on the aperture surface while he is facing towards the viewers, the image then being projected with a proper orientation for the viewers, on a screen behind the operator. The light source can then be placed in the vicinity of the rear edge of the aperture so that the heat from the light source does not cause discomfort to the operator, and so that the dimension of the ellipsoidal mirror in a direction normal to the aperture plane is minimized. It should also be clear that the light can be deflected in the other direction with a deflecting means in the form of an Amici prism or some other deflecting, image inverting means.
Particularly when the projector apparatus includes deflecting means and the projection stage aperture surface is oriented in a substantially horizontal direction, the apparatus may be arranged for mounting in the vicinity of the ceiling in a room, with the aperture surface substantially parallel to the ceiling surface, the projection head then being at a level just under the aperture surface. It is then possible to arrange that the upper boundary of the projected light is adjacent the ceiling surface. The projection stage aperture may include an image-forming liquid crystal display (LCD) screen so that the apparatus serves the function of a television projector.
It should however be evident that such a television projector may very well be shaped without deflecting means and with the projection stage aperture vertical. The projection lens will then be on a level with the lower border of the aperture.
The aperture can advantageously include a plate of material having light transmission substantially adapted to the intensity distribution of the projection light incident on the aperture such that the light passing the plate has substantially uniform intensity.
The invention has been discussed described above in connection with the use of a single ellipsoidal mirror, placed such that the departing light substantially covers the entire projection stage aperture of the apparatus. It should be clear, however, that instead of using a single ellipsoidal mirror, a plurality of smaller ellipsoidal mirror elements can be placed adjacent each other such that they have their first focal points situated substantially in the projection head, with the second focal points having an associated light source. It will then be clear that a light source can be substantially at the common focal point of several smaller ellipsoidal mirror elements. It will also be understood that such a composite mirror comprising at least two ellipsoidal mirror elements can be given a considerably lesser "height", i.e. dimension in a direction normal to the aperture.
It will be further understood that a projector apparatus in accordance with the invention can readily be arranged for projection with very high light intensity. Optionally, as indicated above, the projector apparatus can also be used as a component in a television projection apparatus with very high light strength. In such a case the light source can optionally be such that its colour can be varied. A method of providing such a light source is to connect light emitting diodes (LED's) of different colours together with the aid of optical fibres, preferably relatively thick plastic fibres. The illumination can then be dynamically adjusted to the momentary colour content of the image for obtaining an increase in the colour range.